Wilt diseases occur in various plants, notably woody perennial plants including trees. Wilt diseases involve the vascular system. An infected tree exhibits wilting symptoms with for example leaves, developing discoloration by reason of the disease interfering with water transport in the tree. The leaves typically eventually die and fall off. There is often discoloration or brown streaking in the vascular tissue. Diseased trees may soon die after the onset of the symptoms. Some examples of wilt diseases of trees included Dutch Elm Disease (DED), Fire Blight Disease(FBD), and diseases caused by Verticillium spp., Fusarium spp., and Ceratocystis fagecearum (Oak Wilt).
Since its introduction from Europe during the first half of the twentieth century, Dutch elm disease (DED) has decimated North American elm tree populations, the American elm (Ulmus americana L.) being particularly susceptible to DED.
DED is known to be caused by the fungus Ophiostoma ulmi sensu lato (O. ulmi), which is transported between elm trees by the native and European elm bark beetle. The beetle forms tunnels, also known as galleries, in the bark of the elm tree, and leaves spores of O. ulmi in these tunnels. The fungus then spreads through the tree's water-conducting tubes (vessels). The observable symptoms of DED, namely wilting, yellowing and loss of leaves, and eventually death, are believed to be caused by toxins released by the fungus. One such toxin, which has been associated with DED-like symptoms in American elms, is ceratoulmin (CU).
Fire Blight Disease (FBD) is an aggressive, devastating disease that infects several varieties of trees, including fruit trees, including apples and pears trees, as well as many members of the family Rosaceae. These include the following Genera and species varieties including Amelanchier (serviceberry), Exchorda (pearlbush),Potentilla (cinquefoil), Aroina (chokeberry), Fragaria (strawberry), Prinsepia, Aruncus (goatsbeard), Ceum (avnes), Prunus (apricot, cherry, plum), Chaenomeles (flowering quince), Heteromeles (toyon), Pyracantha (firethorn), Cotoneaster (cottoneaster), Holodiscus (creambush), Pyrus (pear), Cowania (cliff rose), Kageneckia, Raphiolepes (Indian hawthorn), Crataegomespilus, Kerria (Japanese rose), Rhodotypos (jetbead), Crataegus (hawthorn), Malus (apple, crabapple), Rosa (rose) Cydonia (quince), Mespilus (medlar), rubus (brambles), Dichotomanthes, Osteomeles, Sorbaria (false spirea), Docynia, Peraphyllum, Sorbus (mountain ash), Dryas (mountain avens), Photinia (photinia), Spiraea (spiraea), Eriobotrya (loquat), Physocarpus (ninebark) and Stranvaesia. While only affecting members of the rose family, the host range includes over 130 species and nearly 40 genera (Sinclair et al., Disease of Trees and Shrubs, Cornell University Press, 1987). FBD first appeared in the north eastern parts of North America approximately 200 years ago. It has since spread to New Zealand in 1916, England in 1957, Egypt in 1962 and various regions of Europe (Bereswill et al., App. Env. Micro. 58 (1992), pp. 3522-3526, van des Zwet and Bell, HortScience 30(6) (1995), pp. 1287-1291).
Caused by the gram negative bacterium Erwinia amylovora (E. amylovora), the principle symptoms of the disease consist of blackening of the succulent tissues on newly formed shoots, blight of blossoms and fruitlets as well as the formation of cankers that cause the twigs and branches to die back. In fact, the name is derived from the infected plant tissue appearing to be scorched by fire (Barny, Mol. Micro. 4(5) (1990), pp. 777-786). E. amylovora over winters at the margins of cankers from where it emerges by the formation of ooze with the onset of warm weather. Insects, rain splash, birds or humans are some of the vectors for the transmission of the pathogen. However, the most common vector are pollinators such as bees, flies and other insects. Infection courts are stigmas and nectarines, fresh wounds on any plant parts, stomata and lenticels on succulent twigs. It is from these places that the bacteria move rapidly into the vascular system of the host plant resulting in systemic infection and the symptoms associated with the disease (Sincliar et al., ibid.; Bellemann and Geider, J. General Microbiology 138 (1992), pp. 931-940; Momol et al., Plant Disease 82(6) (1998), pp. 646-650; and Bogs et al., Phytopathology 88(5) (1998), pp. 416-421).
As FBD and DED continues to spread and endanger valuable trees, numerous approaches have been tried over the years to eradicate or prevent the spread of DED and FBD in tree populations.
One approach to controlling DED has been to control elm bark beetle populations through the use of pesticides or by cutting infected limbs from elm trees. Another approach is to control or inhibit growth of the fungus by treating infected trees with fungicides or less commonly with antagonistic organisms such as bacteria. Several methods have also been employed to control FBD. One approach is to control the bacteria with antibiotic treatment. Unfortunately, E. amylovora is becoming progressively more resistant to antibiotic treatment with streptomycin (Lindow et al., Phytopathology 86(8) (1996), pp. 841-848; Loper et al., Plant Disease 75 (1991), pp. 287-290; and Moller et al., Plant Disease 65 (1981), pp. 563-568).
However, all of these approaches have disadvantages which limit their effectiveness. In particular, the use of large amounts of chemical pesticides and fungicides is undesirable from an environmental standpoint, particularly in urban areas. Unlike DED, controlling the insect vectors of the disease through the use of pesticides, is not a viable control method for FBD. This is because many of the insect vectors for FBD are the pollinators which are required for the production of fruit. Therefore, pest control of the insect vectors as a means for controlling FBD is not a possible avenue of prevention. In addition, the indiscriminate use of antibiotics to control FBD may have adverse health effects for humans.
Another approach has been to develop strains of elm trees which are resistant to DED, for example by selective breeding. However, such approaches are typically time consuming and do nothing to prevent the spread of DED in existing elm populations. Furthermore, until recently little was known about the mechanisms of DED resistance in elm trees or the means by which O. ulmi kills its host. Therefore, it was unclear whether or not long-term resistance could be bred into elm trees.
Furthermore, the importance of the American elm lies in its umbrella-shaped crown, which makes it a particularly effective shade tree. No other species of elm can compete with the American elm in this respect. Therefore, developing resistance by cross-breeding the American elm with resistant species of elms is useless if the form of the American elm is not maintained.
None of the above approaches has been completely successful in treating or controlling the spread of DED or FBD. Therefore, tree populations remain at risk of being decimated by DED and FBD.
The inventors appreciated that the American elm, which is particularly susceptible to DED, nevertheless produces a defence reaction when infected by a DED-causing fungus. Specifically, it has been shown that elm trees infected with DED produce several sesquiterpene quinones possessing antifungal properties, these compounds being known collectively as "mansonones", Dumas et al., Experiential 39 (1983), pp. 1089-1090. The mansonones known as mansonones "A", "C", "D", "E", "F" and "G" have all been shown to inhibit the growth of strains of O. ulmi. The structural formulas of these mansonones are shown below. ##STR1##
Mansonone accumulation in elms is believed to be triggered by specific compounds produced by O. ulmi which are recognized by the elm tree after it is infected by the fungus. Mansonone-inducing elicitors are present in the culture filtrate, cytoplasm and cell walls of O. ulmi and have been shown to induce production of mansonones in elm tissue cultures, Yang et al., Eur. J. For. Path. 23 (1993) 257-268, Can. J. Bot. 67 (1989) 3490-3497, and Mycol. Res. 98(3): 295-300 (1994).
Although all strains of O. ulmi produce elicitors, it has been found that the less virulent, "non-aggressive", strains of O. ulmi cause elm tissue to accumulate mansonones more quickly and in larger amounts than virulent, "aggressive", strains of O. ulmi (often referred to as Ophiostoma novo-ulmi). This is consistent with the observation that, although all strains can kill susceptible elm trees, the progress of the disease is slower in trees infected by non-aggressive isolates.
Several mechanisms have been proposed to explain the higher virulence of aggressive strains of O. ulmi. It is believed that differential elicitation and/or suppression of mansonone production in elms is at least partially responsible for the higher level of pathogenicity of aggressive strains of O. ulmi. Therefore, it appears that aggressive strains of the fungus may at least partially suppress the production of mansonones in elm trees.
Attempts have been made to use this difference in virulence to induce resistance to highly virulent strains of O. ulmi in susceptible elm trees. Some early inoculation trials using elm seedlings and elm tissue cultures were encouraging. For example, see, Hubbes and Jeng, Eur. J. For. Path. 11 (1981) 257-264, and Hubbes, Naturaliste can. (Rev. Ecol. Syst.), 115: 157-161 (1988). However, a more recent study conducted with European and hybrid elms concluded that, although there is some benefit to be derived from preventatively inoculating elms with O. ulmi or other fungi, there is little reason to think that the method has immediate promise for the control of DED, Sutherland et al., Eur. J. For. Path. 25 (1995) 307-318.
Therefore, extensive research has been conducted into the defence reactions of elms to DED-causing fungi. However, this research has thus far not resulted in any treatments for DED capable of being successfully used on a widespread basis.
The present inventor has appreciated that it may also be possible to induce a protective effect in trees against FBD through the use of the above-noted elicitor. This is a novel approach to control FBD as the DED elicitor is a non-toxic environmentally friendly substance that induces the FBD host's defense mechanism and thereby prevents the FBD pathogen from killing the host.